The present invention relates to a method of manufacturing a thermally conductive article having an integrated surface and the articles produced by such a method. The integrated thermally conductive surface can interface with a heat-generating device (e.g., an electronic component) to dissipate heat from the device.
Electronic devices such as semiconductors, microprocessors, and circuit boards can generate a tremendous amount of heat that must be removed in order for the device to operate effectively. The industry uses thermally conductive compositions to dissipate heat from such electronic components. Typically, such compositions comprise a base polymer matrix and thermally conductive filler material.
For example, McCullough, U.S. Pat. No. 6,251,978 discloses a thermally conductive composition comprising a polymer base matrix (preferably a liquid crystal polymer) and thermally conductive fillers (e.g., aluminum, alumina, copper, magnesium, brass, carbon, and boron nitride).
Conventional thermally conductive compositions can be used in a variety of ways. For example, a heat-generating device (e.g., electronic part) and an adjacent heat-dissipating article (e.g., heat sink) first are secured together by clips, screws, or other mechanical means. Thermally conductive pastes or greases comprising polysiloxane oils loaded with fillers are then smeared onto these components. The thermally conductive greases tend to have initially good film-forming and gap-filling properties. For example, the electronic part and heat sink may have irregular mating surfaces causing small gaps to appear in the interface of these components. Thermal greases tend to seep into these gaps bringing the heat sink and heat-generating device into initial contact with each other. However, it has been found that such thermal greases have poor adhesive properties and will ultimately seep out. This seepage causes air voids to form between the two surfaces resulting in hot spots. Moreover, the mechanical fasteners used to secure the devices may exert excessive pressure and accelerate the seepage. The seeping polysiloxane oils can evaporate and re-condense on sensitive parts of surrounding microcircuits. The re-condensed oils lead to the formation of silicates that can interfere with the microcircuits and cause the microprocessor to fail in operation.
In the case of polysiloxanes and thermoplastic polymers, these materials are typically cast in sheet form and die-cut into desired shapes corresponding to the shapes of the heat sink and electronic part. The resulting pre-formed sheet is attached to the surface of the electronic part, and the heat sink is secured by means of clips or screws. These pre-cut, thermally conductive sheets solve the problems associated with the above-described greases. However, an operator may find it difficult to precisely cut the sheets to specific configurations. Thus, the sheets may not have the proper geometry to provide an optimum pathway for transferring heat from the electronic part to the heat sink. Further, the added step of cutting and manually applying the pre-formed sheets adds cost to the assembly process. The sheets may have non-uniform thickness and vary in their effectiveness to transfer heat. Finally, while these sheet materials are suitable for filling undesirable air gaps, they are generally less thermally conductive than the heat sink member. Thus, these sheets can detract from the overall thermal conductivity of the assembly.
In view of the foregoing problems, it would be desirable to have a method for making a thermally conductive article having an integrated thermally conductive surface, where no further processing or tooling is required to produce the final shape of the article. In addition, the article should form an intimate interface with the heat-generating device and effectively dissipate heat from the device. The present invention provides such a method. This invention also encompasses the articles produced by such a method.
This invention relates to a method of making a thermally conductive article having an integrated thermally conductive surface. The method comprises the steps of: a) providing two molding members in an aligned relationship, wherein a mold cavity is located between the members, b) injecting a first molten thermally conductive composition into the cavity to form a molded body, c) removing a molding member to expose a surface of the molded body, d) injecting a second molten thermally conductive composition onto the exposed surface of the molded body, e) cooling the compositions to form an article having a molded body and an integrated thermally conductive surface, and f) removing the article from the mold.
The first composition comprises a base polymer matrix and thermally conductive filler material. A thermoplastic polymer selected from the group consisting of polyethylene, acrylics, vinyls, and fluorocarbons can be used to form the matrix. Preferably, a liquid crystal polymer is used. The polymer matrix preferably constitutes about 30 to about 60% and the thermally conductive filler preferably constitutes about 20 to about 70% by volume of the first composition.
The second composition comprises an elastomer polymer matrix and thermally conductive filler material. The elastomer polymer can be selected from the group consisting of styrene-butadiene copolymer, polychloroprene, nitrile rubber, butyl rubber, polysulfide rubber, ethylene-propylene terpolymers, polysiloxanes, and polyurethanes. Preferably, the elastomer polymer is a polysiloxane.
The filler material for the first and second compositions may be selected from the group consisting of aluminum, alumina, copper, magnesium, brass, carbon, silicon nitride, aluminum nitride, boron nitride, and zinc oxide.
In one embodiment, the first and second compositions each comprise: i) about 30 to about 60% by volume of a polymer matrix (i.e., an elastomer polymer matrix is used for the second composition), ii) about 25 to about 60% by volume of a first thermally conductive filler material having an aspect ratio of 10:1 or greater, and (iii) about 10 to about 15% by volume of a second thermally conductive filler material having an aspect ratio of 5:1 or less.
The present invention also encompasses thermally conductive articles produced in accordance with the foregoing methods. Preferably, the article has a thermal conductivity of greater than 3 W/mxc2x0 K., and more preferably greater than 22 W/mxc2x0 K.